Windows
Compatible applications
Quick Facts
Basic information about this file format
Extension
.FASL
File Type
Common Lisp Fast Load File
Category
Programming
Best first step
Use compatible software
Quick Summary
A practical overview of the .fasl file format
A FASL (Fast Load) file is a compiled binary object file used by various implementations of the Common Lisp programming language. When a Lisp source file, typically ending in .lisp or .lsp, is processed by the 'compile-file' function, the compiler generates a FASL file containing a representation o…
File extension
.FASL
File type
Common Lisp Fast Load File
Category
Programming
Recommended action
Open with a trusted compatible app
How to open a .fasl file
Start with a trusted app that supports this format
- Check the file extension carefully and confirm that the file name really ends with .fasl.
- Use software that supports this file type. If one app does not work, try another compatible program for the same platform.
- Update the application if the file was created by a newer version of the software.
- Scan files from unknown sources before opening them, especially documents, archives, scripts, installers, and executable files.
What is a .fasl file?
File extension details and usage information
A FASL (Fast Load) file is a compiled binary object file used by various implementations of the Common Lisp programming language. When a Lisp source file, typically ending in .lisp or .lsp, is processed by the 'compile-file' function, the compiler generates a FASL file containing a representation of the code that the Lisp runtime can load and execute more efficiently than raw source code. The primary advantage of using FASL files is the significant reduction in loading time, as the system does not need to re-parse or re-compile the high-level syntax. However, FASL files are usually highly specific to the compiler implementation (such as SBCL, CCL, or Allegro CL) and the specific hardware architecture they were built on. Consequently, they are not intended to be portable across different Lisp environments. They function similarly to bytecode in other languages, acting as an intermediate format that bridges the gap between human-readable source code and machine execution, ensuring that large Lisp applications can be initialized rapidly in production environments. Because they contain compiled machine code or implementation-specific instructions, they are generally not human-readable and require a compatible Lisp environment to be utilized.
Software Compatibility
Programs that may open or work with .fasl files
macOS
Compatible applications
Linux
Compatible applications
Mobile
Mobile applications
Can .fasl files be converted?
Options for saving or exporting this file to another format
In many cases, a .fasl file can be converted by opening it in compatible software and using an option such as Save As, Export, or Convert. The available output formats depend on the file type and the application used.
- Use the original software that created the file when possible.
- Check whether the application supports exporting to common formats.
- Be careful with online converters when working with private, business, or sensitive files.
- If conversion fails, the file may be corrupted, incomplete, encrypted, or not actually a .fasl file.
Why can't I open a .fasl file?
Common reasons this file type may not open correctly
Wrong software
The file may require a specific program that supports the .fasl format.
Incorrect extension
The file extension may have been renamed manually, even though the actual file format is different.
Corrupted file
The file may be incomplete, damaged during download, or transferred incorrectly.
Outdated app
Your software may need to be updated before it can open newer versions of this file format.
Frequently Asked Questions
Common questions about .fasl files
To open a .fasl file, use software that supports this file format.
- Steel Bank Common Lisp (SBCL)
- Clozure CL
- Allegro CL
- LispWorks
- GNU CLISP
Open the file in compatible software and look for options such as Save As, Export, or Convert. If no export option is available, the file may require a specialized converter or the original application that created it.
- Verify that the file extension is correct.
- Install or update compatible software.
- Try another application that supports this format.
- Check whether the file is corrupted, incomplete, or encrypted.
A file extension alone does not guarantee whether a file is safe or unsafe. Only open files from trusted sources, scan unknown files before opening them, and be especially careful with executable, script, archive, and document files.
File safety tips
Use caution with unknown files
Only open files from trusted sources. Before opening a .fasl file, scan it with antivirus software, verify that the file extension is expected, and avoid running unknown executable, installer, macro-enabled document, or script files.